DAFS: avoid saving fileserver state when panicing

DAFS: avoid saving fileserver state when panicing

When ShutDownAndCore is called with dopanic=1, avoid trying to save
fileserver state. When we are panic'ing it is very possible that the
thread that called ShutDownAndCore is one of the background threads we
are waiting for, or that the calling thread is holding H_LOCK. Since we
are panicing, the fileserver state is probably not consistent anyway, so
just avoid trying to save state altogether, and avoid a possible
deadlock.

Change-Id: If727808bfdfda74a4ec0e65b27ef9c77fbee4aae
Reviewed-on: http://gerrit.openafs.org/997
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>